x - 3 = (88 + x + 85 + 90 + 80)÷5
5(x - 3) = 343 + x
5x - 15 = 343 + x
5x - x = 343 + 15
4x = 358
x = 358 ÷ 4
x = 89.5
答：数学成绩是 89.5 分.